Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Originally Posted by Yamamaya42

I did restuff some of the original cans, and when I pulled the guts from them, they were as dry as the Sahara Desert inside...

That is why they are called "Dry Electrolytics"!!! Lol.

[Yamamaya42]

Originally Posted by Penthode

That is why they are called "Dry Electrolytics"!!! Lol.

these caps were not the "dry" type.
but became so over time.

"the electrolyte inside the capacitor slowly evaporates away over time,"

https://www.effectrode.com/knowledge...citor-rebuild/

[Electronic M]

Originally Posted by Penthode

That is why they are called "Dry Electrolytics"!!! Lol.

Wrong. They're called dry Electrolytics as they are not a can of electrolyte fluid with a thin positive electrode in the center like the set Electrolytics used in 30s early 30s Philco cathedral radios.

If you tear open a good dry Electrolytic of any vintage at the very least there should be some electrolyte moisture on the foil/dielectric roll. If there isn't it's dead or about to be.

[Penthode]

Originally Posted by Penthode

That is why they are called "Dry Electrolytics"!!! Lol.

You can understand why I never made it as a stand up comedian. Nobody laughs at my jokes.

[Penthode]

Well it has another 5 hours which brings it up to a total of 30.

This evening I explored the slight smear. I first discounted RF/IF alignment and checked components is the video amplifier. The peaking coils are all good and the load resistors are within tolerance. I temporarily substituted a new 0.05 uF coupling capacitor between 2nd detector and video amplifier input and no change. I used my trusty grid dip meter to check peaking coil resonance. All appeared normal.

I decided to investigate the IF alignment using the grid dip meter Before even even touching any adjustment, injecting a modulated signal at the mixer in the tuner, I could check the sound trap and visually check the IF in a crude way. The traps were fine and you could listen as the dip matched the sound discriminator "zero" at 21.25 MHz. I did notice the video carrier appeared a tad high on the nyquist slope. It certainly helps to have no AGC as the test was quite simple. Typically for virtually all other sets with AGC, I would put a DC supply on the AGC line to prevent AGC interaction.

I ultimately found the last stagger tuned IF stage tuned to 26 MHz may be pushing up the frequency response around the video carrier. I tried swapping the 6AG5 in the last IF stage with a NOS RCA tube which appeared to reduce the video carrier on the nyquist slope. The multiburst 3.0MHz bar appeared more prominent and when connecting back the DVD player, the picture was noticably sharper.

I have an old but good AVO Mark III transconductance tester and I am inclined to test the 6AG5 tomorrow. I also found that the sync separation is not as good as I would anticipate: at normal contrast, sync separation is fine but the contrast adjustment range is narrow before the picture bends. The 721TS uses the first iteration of the famous RCA 'Synchroguide' circuit but without the sine stabilization. So the performance I see may be normal for this pruned back design. But I will check the 6SN7 and the sync circuitry later tomorrow evening.

[Penthode]

Well it has another 5 hours which brings it up to a total of 30.

This evening I explored the slight smear. I first discounted RF/IF alignment and checked components is the video amplifier. The peaking coils are all good and the load resistors are within tolerance. I temporarily substituted a new 0.05 uF coupling capacitor between 2nd detector and video amplifier input and no change. I used my trusty grid dip meter to check peaking coil resonance. All appeared normal.

I decided to investigate the IF alignment using the grid dip meter Before even even touching any adjustment, injecting a modulated signal at the mixer in the tuner, I could check the sound trap and visually check the IF in a crude way. The traps were fine and you could listen as the dip matched the sound discriminator "zero" at 21.25 MHz. I did notice the video carrier appeared a tad high on the nyquist slope. It certainly helps to have no AGC as the test was quite simple. Typically for virtually all other sets with AGC, I would put a DC supply on the AGC line to prevent AGC interaction.

I ultimately found the last stagger tuned IF stage was tuned to 26 MHz. I tried swapping the 6AG5 in the last IF stage with a NOS RCA tube which appeared to reduce the video carrier on the nyquist slope. The multiburst 3.0MHz bar appeared more prominent and when connecting back the DVD player, the picture was noticably sharper.

I have an old but good AVO Mark III transconductance tester and I am inclined to test it tomorrow. I also find that the sync separation is not as good as I would anticipate: a normal contrast it is fine but the range is small before the picture bends. The 721TS uses the first iteration of the famous RCA 'Synchroguide' circuit but without the sine stabilization. So the performance I see may be normal for this pruned back design. But I will check the 6SN7 and the sync circuitry later tomorrow evening.

[Penthode]

Here is a shot of the grid dip meter coupled to the tuner mixer tube for the frequency response check. I actually rested the coil on the tube as I swung the calibrated dial. Accuracy is reasonable and the trap null was indicated on the dial at 21.25 MHz. I also see the original green sealing lacquer still on the adjustments which suggests it has never been touched.

[Penthode]

Running a so far an hour this morning checking the picture and sound quality. Sound is excellent with no buzz. Picture sharper since the 3rd IF tube change yesterday. Haven't touched the alignment: still has the factory settings.

[Electronic M]

Originally Posted by Penthode

You can understand why I never made it as a stand up comedian. Nobody laughs at my jokes.

Perhaps I should take things less seriously and have a chuckle.

I've been banging my head on the wall with an early tube MPX decoder in a radio that will recognize the 19KHz from a generator but not a station...I have to complete this and some design improvements to the set in a few days. The pressure is making me a bit edgy.

That said your experience leads me to have completely different feelings about vintage capacitors than you, and whats left of my understanding of college engineering reliability statistics tells me I shouldn't change my mind.

[Penthode]

Originally Posted by Electronic M

.... your experience leads me to have completely different feelings about vintage capacitors than you, and whats left of my understanding of college engineering reliability statistics tells me I shouldn't change my mind.

Well I am a professional Electronics Engineer. I know what I have read and been told. In a professional sense, keeping old capacitors like this is wrong. But heck this is a hobby about extreme vintage televisions. The owners of this set put it away in 1960 and forgot about it because by 1960 it was if not obsolete, quaintly old fashioned and dated.

For historical reasons, I want to keep as much originality to it consistent with it working reasonably reliably. Perhaps even the 60 years it sat unused help preserve the electrolytics? Who knows? All I can say is that the capacitors show the correct leakage and capacity and the set will operate for a dozen hours at a time without getting warm.

As for the other components, so long a the picture and sound remain excellent as the set currently does, what is the issue

From purely a hobby perspective and as I wish to keep this set and because I am technically inclined, I am very happy that I have such a good working set with much of its originality maintained
I am going to keep the set running and will update this column as the set approaches 100 hours on time.

Meanwhile I gave an old 630TS someone tried to repair and gave up on. I am going to give it a go and try to clean up the wiring.

[init4fun]

Tis a sad fate indeed suffered by all too many rectifiers .....


I trust my x wife 100 times more than I trust any electrolytic cap that's as old as I am

[Penthode]

Originally Posted by init4fun

Tis a sad fate indeed suffered by all too many rectifiers .....


I trust my x wife 100 times more than I trust any electrolytic cap that's as old as I am

The reason you trust your wife more is because you are kinder to her than your electrolytics! If you read earlier thru the thread, the patient and proper reforming of the dielectric is the key. How long running time will it take to convince you the capacitors are okay?

[init4fun]

Originally Posted by Penthode

The reason you trust your wife more is because you are kinder to her than your electrolytics! If you read earlier thru the thread, the patient and proper reforming of the dielectric is the key. How long running time will it take to convince you the capacitors are okay?

I was more or less joking with you , if you noticed I said "my x wife" , no kinder there my friend .

Kinda like your "dry" joke , I guess humor & electrolytics just don't mix


and yes , I am interested in how long the reformed caps live .

[Eric H]

I wish I could find the pictures I have of some old caps I took apart.

When I say rotted I mean exactly that, if you unroll the foil there are areas where the aluminum has been eaten away, with gaps in the foil, there are often lumps from the crystals that have formed that leave dents on the paper insulator, I would guess it's these dents, if they get bad enough, that press through the insulator and cause shorts. I have seen several caps where the connecting leads between the foil and the lugs have corroded completely in two causing an open.

And of course I have opened some 40's caps up to find they are still wet and pristine inside, but that is the exception. I imagine the type of storage and the length of time they have been inactive plays a part.

I'm not trying to be contrary, it's an interesting project to try minimal repairs, but I think it is a gamble that they will continue to work, not a big deal since this is a hobby and if it craps out it's not a big issue.

I'm an amateur hobbyist, so it's usually easier for me to just change them and be done with it rather than wonder if that odd little problem I'm having is being caused by an old capacitor.

[dtvmcdonald]

I have always been of the impression that electrolytics that are bone dry
cannot out damn bug! OUT! you damn cockroach get out of my keyboard reform.
Reforming is supposed to take solvent.

I have seen caps that were bone dry and had normal capacitance and
took rated voltage, but I assumed that they were that way before going dry.
What think you people.

I hope you fry, damn bug ... wait, no, that might hurt the keyboard. OUT!
Damn coronavirus, keeping the exterminators away!